DEVELOPMENT OF THE HUMAN BEING 
ward. They resolve themselves into three categories, 
called in the language of embryology, the ectoderm, the 
entoderm, and the mesoderm (Fig. 3). 
Of course, the new creature has not yet given any 
recognizable visible signs of the wise head, the strong 
Entoderm of primitive gut Amnion 
Ly 
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qm | 
[sd i | 
fos < i i 
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Somatic mesoderm 
Splanchnic mesoderm 
Wall of allantois 
Chorionic mesoderm 
Chorionic ectoderm 
Uterine epithelium 
Tunica propria of uterus 
Fic. 3. Diagram of the fetal membranes and allantoic placenta of a pig embryo, 
showing the three primary germ layers, ectoderm, entoderm, and mesoderm, 
from which all tissues and organs of the body are derived. After Prentiss and 
Arey 
body, the nimble legs, which later will appear. The three 
primitive divisions which we now speak of are much 
simpler; for the ectoderm is merely the original source of 
that which develops the outer skin, with hair and nails, the 
lining of the mouth and nose, the nervous system, and the 
[29] 
